Centrifugal casting coating composition for centrifugal molds and method of coating molds



Dec. 30, 1952 A R. c. MYERS 2,623,809

CENTRIFUGL CASTING COATING COMPOSITION FR CENTRIFUGAL HOLDS AND METHODOF' COTING MOLDS Filed May 2 1949` 2 SHEETS-SHEET l Dec. 30, 1952 Q QMYERS 2,623,809

CENTRIFUGAL CASTING COATING COMPOSITION FOR CENTRIFUGAL MOLDS AND METFJDOF COATING MOLDS Filed May 2, 1949 2 SHEETS--SHEET 2 INVENTOR.

` foer @@8715 Bgm, M, WMV# @yu- ATTORNEYS.

Ratented Dec. 3%, lfai CENTRIFUGAL CASTING COTING COMPU- SITION FORCENTRIFUGAL MLDS ND METHOD F COATING MOLDS Robert C. Myers, CambridgeCity, Ind., assigner to Perfect Circle Corporation, Hagerstown, Ind..

a corporation of Indiana Application May 2, 1949, Serial No. 90,904

14 claims. l

The invention relates to centrifugal casting of tubular metal articlesin permanent metal dies, and particularly to articles of cast iron. Thegeneral object of the invention is to provde a novel method of and moldfor making centrifugal castings, resulting in castings which are free ofdefects such as pin holes, dents or inclusion of foreign matter, andwhich have a relatively even exteriorv surface conforming closely to adesired'diameter, so that the castings may be machined to a finishedsize with a minimum removal of stock. i

Another important object is to provide a novel method of and mold formaking centrifugal castings of the character referred to in thepreceding object, which permit of a high rate of production for thecastings. i

A'further object is to Aprovide a novel method of and mold for makingcentrifugal castings of cast iron, by which a desired metallurgicalstructure may be attained.

Still another Objectis to provide a novel reiractory coating for theinner surface of the permanent metal dies.

Other objects and advantages will become apparent from the followingdescription taken in connection with the accompanying drawings, inwhich:

Fig. l is a diagrammatic longitudinal sectional view of a die beingsprayed with a coating composition, to illustrate one of the steps of mynovel method.

Fig. 2 is a view similar to Fig. l but showing the coating completed andthe molten metal being supplied to the die.

Fig. 3 is a photographic view of the inner surface of the coatingmagnified six times.

Fig. 4 is a photographic view of the outer surface of a casting made bythe method disclosed herein, the surface being magnied six times.

In the art of making ytubular metal castings in permanent cylindricalmetal dies, the dies are usually provided with some sort of refractoryliner, both for the purpose of protecting the dies from the action ofthe heat of the molten metal and for the purpose of preventing too rapidcool ing of the metal. If gray cast iron were the metal being cast, suchrapid cooling would result in the formation of chills in the castings,which would render the metallurgical structure of the casting unusablefor most purposes.

One type of liner is formed of ordinary molding sand placed in the dieto form a liner of sufficient thickness to insulate the casting from themetal die. The resulting casting will be similar to an ordinary sandcasting made in the usual stationary mold and will have many of thedisadvantages thereof. Thus, the exterior of the casting will berelatively rough and irregular and will include particles of sand, sothat a substantial amount of metal must be removed from such a castingto'bring it to a finished dimension. Another diiculty with such a methodis that the rate of production is relatively low. Such a method,however, has the advantage, in making cast iron castings, that thecooling rate of the metal is such that a desired metallurgical structuremay be obtained.

Another method of making castings centrifugally in permanent metal dieshas been proposed. in which the die is lined with a coating consistingof a powdered refractory material such as silica flour and a binder suchas bentonite applied in the form of an aqueous suspension, the method ofapplication being by means of a spray to build up the coatingincrementally. In this method, the traction on theimetal causing it torotate with the die is dependent upon the roughness of the inner surfaceof the coating. Consequently, great care must be exercised in applyingthe coating to attain such surface roughness, and

the incremental application of the material isnecessary to this end.Moreover, such roughness necessarily results in a surface on the castingof similar roughness, and to machine such a cast ing to a finisheddimension requires considerable removal of stock.

It frequently occurs, in making castings in a die provided with thistype of liner, that the castings have the defects of pin holes and-dentsin their outer surfaces. These defects may be due to gases forming whenthe molten metal contacts the liner or may be due to traces of moistureremaining in the coatingr when the metal is poured. Of course, completedrying out of the coating would eliminate this last mentioned cause, butmoisture may be trapped in the coating by the drying of the surroundingcoating material and a longer time is required to effect the drying out.

A coating of this sort, while it may have sufficient strength to.prevent its being broken up by the pouring of the molten metal, does nothave sufficient strength to remain intact when the casting is ejectedfrom the die. Such ejecting is performed when the die is heldstationary, so that the centrifugal force is not present to hold thecoating against the inner surface of the die, as is the case when themetal is being poured. During the ejecting of the casting, this coatingbreaks up and tends to jam in the clearance between the casting and thedie, thus making it difficult to effect such ejectment.

The present invention relates to a coating of this last-mentioned typebut distinguishes therefrom in its composition and in its ultimatecharacteristics, as well as in the method of makingl centrifugalcastings. Thus, while the present coating composition may be, andpreferably is. sprayed on the die, no great care-need be exercised in soapplying it. The composition, almost regardless of the manner in whichit is aD- plied, inherently results in a coating having .tractionfeatures causing a rapid pick-up of the molten metal when poured intothe die for rotation therewith and yet provides a surface on the castingwhich requires much less stock removal, to bring it to a finisheddimension, than is necessary with the two processes heretofore referredto. The coating provides for rapid evaporation of .the moisture contentof the composition, so that a high rate of production may be obtained,with the castings free of defects such as pin holes or otherimperfections. The coating provided by the present composition may havesufficient thickness to give the desired insulating qualities which, inconnection with the temperature of the die when the metal is poured, andwith the temperature of the metal at that time, provides a controlledrate of cooling of the casting so that, in the case of cast iron, adesired metallurgical structure may be attained.

Another important aspect of the present coating is the fact that it hassufficient strength to remain intact throughout the entire castingprocess.` Thus, it not only withstands the action of the molten metalwhen the latter strikes the coating, but also remains intact during theejectment of the casting from the die. The coating composition, ofcourse, is applied to the die when the latter is rotating at castingspeed, and the composition is thus distributed and held in an evencoating on the interior of the die during drying. As a result, the driedcoating necessarily is held in engagement with the liner, but there isnothing inherent in the coating which causes it to adhere to the die.The strength of the coating is such that it may be said to beself-sustaining. This lack of anything inherent in the coating to causeit to adhere to the die facilitates the removal thereof from the diealong with the casting. Thus, because of the strength of the coating, itis removed from the die without breakage along with the casting. Infact, it is so free of any quality of adherence to the die that itfacilitates the removal of the casting. The complete removal of thecoating with the casting thus leaves the die clean and ready forre-coating.

The coating composition of the present invention is, as heretoforestated, of a character which includes a powdered refractory material anda binder in an aqueous mixture. Preferably, the refractory material issilica flour and the binder is bentonite. These produce a desirablecomposition and are readily obtainable and for these reasons -arepreferred, but the invention is by no means limited to these specicmaterials. To this mixture is added a wetting agent to reduce thesurface tension of the water in the mixture.

From this, surprising results are obtained. The h reduction in surfacetension of the Water provides a more homogeneous mixture, probably dueto the increased wetting action on the surfaces of the particles of therefractory material and the consequent more thorough dispersion ordistribution of the binder on the particles, thus obtaining full benefitof the binding action of the binder and providing strength in theultimate coating. This homogeneous character of the composition may beexplanatory of the fact that the present coating is found to be muchstronger than coatings of this type heretofore known. An examination ofthe coating after it is dried shows it to have a spongy structure, whichno doubt explains its excellent insulating properties.

Another remarkable characteristic of the present coating is the natureof the inner surface of the coating after it is dried. This surface,while it is generally smooth and even in its major aspect and thusradically different from the rough coating heretofore thought necessary,obtains the necessary traction of the metal by having a multitude ofsmall cavities extending generally radially into the body of thecoating. Such cavities may be described as relatively deep and narrow.and are such that, when the molten metal is poured into the die, themetal enters such cavities to form elongated thin spines on the exteriorof the casting. Such spines are rapidly cooled because of their smallmass and the fact that the heat therein may be rapidly lost. The spinesthus tend to anchor the molten metal to the coating to cause the metalto rapidly pick up the r0- tational movement of the die so that themetal is quickly subjected to the centrifugal action.

The presence of such spines on the casting provides a number of otheradvantages. Thus, they tend to reinforce the coating and cause it tormly adhere to the casting. In ejecting the casting from the die, suchfirm adherence of the coating to the casting and the lack of anyadherence between the coating and die cause the coating tobe ejectedalong with the casting without any breakage of the coating. In fact, thecoating is so firmly tied to the casting by the spines that, even in thesubsequent handling of the castings when they are piled on trucks fortransport away from the casting apparatus, the coating remainssubstantially intact on the castings in spite of the bumping and rubbingto which the castings are subjected in such handling.

The explanation of why such cavities are formed in the coating lies inthe use of the wetting agent in the composition, since experiments haveamply shown that the cavities are obtained only when a wetting agent isemployed. and when the wetting agent is omitted, no such cavities areformed. The presence of the wetting agent reduces the surface tension ofwater in the composition and thus increases the ability of the water topenetrate the spaces between the particles of silica flour readily. Thedie. of course, is rotating at casting speed when the coatingcomposition is applied thereto. The specific gravity of the1 solids inthe coating composition is greater than that of water so that, under thecentrifugal action, the water will tend to be forced to the innersurface of the' coating. The reduced surface tension of the waterfacilitates this action, so that the water more readily moves to theinner surface. There is thus less chance of any traces of moisture beingentrapped in the coating to cause defects such as pin holes and dentsinthe casting. The separation of the water from the solids in thismanner together with the subsequent evaporation thereof may result inthe formation of the cavities noted in the coating.

This separating action of the water by the centrifugal force may betermed a wringing action, and its occurrence is confirmed byobservations made when a coating composition of this character isapplied to a die. When the coating composition is first sprayed, theinner surface, which at rst has a milky white appearance, quicklychanges to a characteristic shiny appearance `caused by thepreponderance of water at such inner surface.

The die, prior to application of the coating. has been heated to atemperature correlated to other factors to attain a desiredmetallurgical structure, as hereinafter described, and such temperatureis well above the boiling point of water so that, as the water is thuswrung out of the composition, it freely and rapidly evaporates. Thisevaporation is so fast that a heavy mist may be observed within the die,which clears up in a few moments. The drying. however. is thorough andbecause of its rapidity permits Pouring of the molten metal into the diesoon after the coating has been placed therein. Production, therefore,may be maintained at a high rate. The present coating gives ampleprotection to the die from the heat of the molten metal and thus delaysand decreases neat checking of the die. The die life with this form ofcoating is thus increased over what has heretofore been possible.

Since the spines on the casting cause a rigid adherence of the coatingto the casting and prevent the coating from being `broken duringejectment of the casting from the die. such ejectrnent may beaccomplished merely by tilting the die endwise and vibrating it. Thecasting thereupon slides out easily of its own accord. While, in someinstances in the past, it has been felt necessary, with the previouscoating, to provide tapered dies to facilitate the ejectment, the easewith which the castings slide out of the die in the present instancepermits use of straight dies. This, of course, decreases the amount ofmetal that must be removed in machining a casting.

As heretofore mentioned, the coating tends to adhere to the casting evenduring subsequent handling of the casting after removal from the die.but the removal of the coating may be readily accomplished by tumblingor by shot-blasting or by a combination of the two operations. Suchoperations, however, are found to have an additional advantage in that,when the coating is thereby removed, the spines are likewise removed.The spines, of course, are rapidly chilled, as heretofore described, andconsequently are relatively brittle. In view of their thinness, thetumbling or shot-blasting, therefore, readily knocks off the spinesalong with the coating. Since the inner surface of the coating isrelatively smooth except for the cavities, the resultant surface of thecasting after this operation is quite regular and even and thus closelyapproaches a truly cylindrical surface. Consequently, a very smallamount of metal need be removed in subsequent machining operations tobring the casting to a desired size. In most of the castings made by thepresent process, not more than .030 of an inch of stock need be removedto clean up the surface. This advantageously compares with the roughnessof the exterior surface of castings made by the prior processes and thelarger amount of metal that must be removed from a casting made by suchprior processes.

To briefly illustrate the preferred manner of coating the die with thepresent coating compo- 'one end having a central aperture substantiallysmaller than the internal diameter 'of the die proper so that neitherthe coating nor the molten metal will run out oi the end, thecentrifugal force, of course, acting to hold the coating as well as themolten metal against the side wall of the die, at the other end of thedie is a similar cap, but such cap includes a" removable portion itwhich is disengaged from the die proper after the casting has Ibeencompleted so that the iatter may be ejected endwise from the die. Fig. lshows a preferred means for applying the coating, which, in thisinstance, comprises a pipe i3 through which the coating composition issupplied under pressure, and which is movable endwise of the die and hasa nozzle iii at its end to spray the coating composition generallyradially onto the inner surface oi the die. In the preferredarrangement, the pipe it moves inwardly through the i'ull length of thedie and when it is at the left end of the die as shown in Fig. 1, thespraying of the coating composition is started with the pipe iii beinginovedi gradually toward the opposite end to apply :a uniform amount ofcoating composition throughout the length oi the die. The centrifugalaction, of course. effects a uniform distribution of the coating so thatthe exact manner of spraying the coating on the die is ci no greatimportance. However, in the present instance, the spray is shown in aian-like form extending :in a plane through the axis of the die. Theresultant coating is indicated at iii.

Fig. 2 illustrates the die after the coating has been completed and whenthe molten metal is being poured therein. In the present instance, suchmetal is introduced into the die through a spout it which enters the diethrough the central aperture in the cap iii. The centrifugal action ofthe die quickly distributes the metal evenly over the entire innersurface of the coating, as indicated at il, the quantity of metaldetermining the thickness of the casting. While the die shown in thedrawings is adapted for rotation on a horizontal airis and such positionis preferred, the invention is not limited to use of a die in suchposition.

Fig. 3 is a magnified view of the inner surface of the coating after ithas been dried and clearly shows the cavities that are formed by thewringing out of the water due to the effect of the wetting agent, whileFig, i is a similar view of the outer surface of the casting formed inthe coating. In this latter ngure, the spines on the casting are clearlyapparent.

The coating composition, as heretofore mentioned. consists essentiallyof a powdered refractory material and a binder in an aqueous suspensionwith a wetting agent added to reduce the surface tension of the water,by which the many advantages of the present process are obtained. Thepreferred refractory material is silica flour,

the neness of which should be such as not to interfere with the sprayingof the coating composition. Preferably, silica flour of sieve size isused. Graphite, which is known to be refractory, and mixtures ofgraphite and silica flour may be used in place of the pure silica flour.The binder is such that it provides the necessary adhesive Qualities toresult in a self-sustaining coatingr and, of course, should not containany material which would tend to cause gases to evolve when the coatingis contacted by the aeaasoe molten metal. Because of its availability,ease of handling and strength, bentonite is the binder which ispreferred, but any binder such as the naturally occurring clays, may beused if it has the foregoing qualities.

The exact consistency of the composition is not important except that itmust be capable of being readily sprayed onto the die, nor is the exactratio of refractory material to binder of great import. In a preferredmixture, the silica our may comprise 90% to 98% of the solids,

while the bentonite may constitute the remainder, thus being within therange of 2% to 10%. The amount of water added to these solids ispreferably sufficient to bring the Baurn reading to within the range of55 to 65".

As to the wetting agent, since its effect in the casting operation isphysical and not of a chemical nature, any of the well-known wettingagents may be used. Thus, in actually practicing the invention, thewetting agent known in the trade as Orvus, which is a sodium salt oftechnical lauryl sulfate, as well as the wetting agent known as AerosoloT-C, which is di oct-vl sodium sulfosuccinate, have both been found tebe satisfactory. Any of the wetting agents may be used in amounts notmore than a few tenths of a per cent, for example, 4 cubic centimetersper gallon of the aqueous composition. The invention, however, is by nomeans limited to the specific quantities of the various ingredients.

One of the features of the invention lies in the control which may beexercised over the metallurgical structure of the metal, particularly inthe case of gray cast iron. In the use of thin coatings heretoforeknown, an undesirable metallurgical structure of such cast iron wasalmost invariably encountered. This was due chiefly to the rapid coolingrate of the metal and resulted in a structure having dendritic graphiteand a matrix of primary ferrite. Such castings were also exceedinglyhard because of the chilling action. While annealing would reduce thehardness, it would not correct or eliminate the presence of thedendritic graphite and the ferrite.

With the present invention, a coating may be placed on the die which,while thin compared to sand linings, nevertheless has sufficientinsulating characteristics to result in controlled cooling of thecasting. With such insulating characteristics possible, by control ofthe temperature of the die at the time of pouring and control of thetemperature of the molten iron, a desirable metallurgical structure maybe obtained. Thus, in a gray cast iron, the graphite may be random andnondendritic and is flaky and not powdery. The matrix obtained may bepearlitic or ner and relatively free from either free primary.ferrite'or graphite segregations.

In a preferred casting process, the initial die temperature may rangefrom 700 F. to l000 F., while the pouring temperature of the iron may bein the neighborhood of 2700" F. With a coating thickness of .040 to .060of an inch, the desired metallurgical structure will be obtained.

The better insulating character of the die coating. compared with thecoatings heretofore known, may be explained by the spongy characterthereof through the use of the wetting agent. As heretofore mentioned.the reduction of surface tension of the water in the coating compositionby the wetting agent results in a more homogeneous structure in thecoating and the latter is more completely freed of moisture than theprevious coatings where apparently traces of moisture frequently remainin the coatings to be eliminated only by evaporation. With the presentcoating, the water appears to be almost completely wrung out of thecoating by the centrifugal action and the dried coating thus has a morespongy structure than has heretofore been obtainable. rThis provides thegreater insulating characteristic of the present coating and,consequently gives better control of the cooling rate of the casting toattain the desired metallurgical structure, as aforementioned.

I claim:

l. In the method of making centrifugal metal castings in a cylindricaldie, the steps of heating the die to a temperature of about 700 F. toabout 1000U F., coating the interior of the die during rotation with anaqueous coating composition of a powdered refractory material, aclay-type binder, and a wetting agent.

2. In the method of making centrifugal metal castings in a cylindricaldie, the steps of heating the die to a temperature of about 700 F. toabout 1000c F., applying to the interior of the die during rotation anaqueous composition of bentonite, silica our and a wetting agent, anddrying the composition during rotation of the die to form a stronginsulating coating on the interior of the die.

3. 1n the method of making centrifugal metal castings in a cylindricaldie, the steps of heating the die to a temperature of about 700 F. toabout 1000 F., coating the interior of the die during rotation thereofwith an aqueous composition of powdered refractory material, a claytypebinder and a wetting agent, with the heat of the die evaporating thewater from the composition, the wetting agent causing the formation,=bythe evaporation, of cavities in the coating df such form that elongatedthin spines are formed on the casting by the molten metal enteri thecavities, and pouring a predetermined quanty of molten metal into thedie during rotatifgnthereof after such evaporation has occurre` 4. lnthe method of making centrifugal metal castings in a. cylindrical die,the steps of heating the die to a temperature of about 700 F. to about1000o F., coating the interior of the die during rotation thereof withan aqueous composition of powdered refractory material, a clay typebinder, and a wetting agent. with the heat of the die evaporating thewater from the composition, the wetting agent causing the formation, bythe evaporation, of cavities in the coating of such form that elongatedthin spines are formed on the casting by the molten metal entering thecavities, pouring a predetermined quantity of molten metal into the dieduring rotation thereof after such evaporation has occurred, andelecting the casting endwise from the die with the spines securing thecoating to the casting for removal therewith and strengthening thecoating to prevent breakage thereof during such removal.

5. The method of centrifugally casting cast iron sleeves, whichcomprises heating a cylindrical metal die to a temperature of about '700F. to about 1G00D F., coating the interior of the die during rotationthereof with an aqueous composition consisting essentially of bentonite,silica flour and a wetting agent with the heat of the die evaporatingthe water in the compound, to provide on the interior of the die aninsulating coating of substantially .O40 to .060

of an inch in radial thickness, pouring molten iron at a temperature ofabout 2700 F. into the die during rotation thereof in a quantitysufficient to provide a sleeve of desired wall thickness, and retainingthe sleeve in the die until cooled below its critical temperature, thecooling of the iron being retarded by the temperature of the die and bythe insulation provided by the coating to give the iron a metallurgicalstructure in which the graphite is non-dendritic and the matrix ispearlitic and substantially free of ferrite.

6. The method of making centrifugal castings, which comprises coatingthe interior of a die, while heated to a temperature of about 700 F. toabout 1000 F. and during rotation thereof, with an aqueous coatingcomposition of powdered refractory material, a clay-type binder and awetting agent with the heat of the die evaporating the water in thecomposition to form cavities in the coating of such form that elongatedthin spines are formed on the casting by the molten metal entering thecavities, pouring a predetermined quantity of molten metal into the dieduring rotation thereof, said spines being rapidlycooled by theirproximity to the die and thus becoming brittle, removing the castingtogether with the coating from the die, and tumbling the casting toremove the coating and simultaneously to remove the spines to render thecasting relatively smooth, whereby the amount of stock removal insubsequently machining the casting is held to a minimum.

7. A permanent metal die for centrifugal casting, having an insulatingrefractory coating formed on the inner surface thereof by applying tothe die when heated to a temperature of about 700 F. to about 1000o F.and while being centrifused an aqueous composition of powderedrefractory material, a clay-type binder and a wetting agent.

8. A coating composition adapted to be applied to a rotating permanentmetal die for centrifugal casting, consisting essentially of powderedrefractory material, a. clay-type binder, water and a wetting agent.

9. A coating composition adapted to be applied to a rotating permanentmetal die for centrifugal casting, consisting essentially of ahomogeneous mixture of powdered refractory material, a clay-type binder,water and a wetting agent, the coating composition having a uniformdispersion of the ingredients to produce a relatively strong coating inthe die.

10. A coating composition adapted to be applied to a rotatingcentrifugal casting die, con sisting essentially of a homogeneousmixture of powdered refractory material, a clay-type binder,

fill water and a wetting agent, the water being sumcient to permit themixture to be freely sprayed on the die.

1l. A coating composition adapted to be applied to a rotatingcentrifugal casting die, comprising a mixture of bentonite, silica flourand water, the silica flour comprising .from 90% to 98% of the solidsand the bentonite the remainder, the water when added to the solidsbeing in sulficient quantity to bring the Baume reading of the mixturewithin the range of 55 to 65, and a wetting agent to give the mixture ahomogeneous character.

12. A coating composition adapted to be sprayed on a rotating metal diefor centrifugal casting, consisting essentially of bentonite, silicaflour, water, and a wetting agent.

13. A permanent metal die having a cylindrical cavity for centrifugallycasting hollow cylindrical bodies, having an insulating refractorycoating on the inner surface thereof, the coating having a relativelysmooth inner surface and having cavities with generally circularopenings at its inner surface and generally isolated from one anotherand extending into the body of the coating to form elongated thin spinesof the metal on the outer surface of the casting.

14. A metal casting centrifugally formed in a die comprising an annularhollow cylindrical body of metal having spines on its outer cylindricalsurface generally isolated from one another and extending generallyradially, and a refractory coating having a substantially uniformthickness at least equal to the maximum length of said spines andinterloclied with said spines whereby said coating is secured to thebody for removal of the coating from the die along with the body.

ROBERT C. MYERS.

REFEREBNCES CITED The following references are of record in the lle ofthis patent:

UNITED `STATES PATENTS

2. IN THE METHOD OF MAKING CENTRIFUGAL METAL CASTINGS IN A CYLINDRICAL DIE, THE STEPS OF HEATING THE DIE TO A TEMPERATURE OF ABOUT 700* F. TO ABOUT 1000* F., APPLYING TO THE INTERIOR OF THE DIE DURING ROTATION AN AQUEOUS COMPOSITION OF BENTONITE, SILICA FLOUR AND A WETTING AGENT, AND DRYING THE COMPOSITION DURING ROTATION OF THE DIE TO FORM A STRONG INSULATING COATING ON THE INTERIOR OF THE DIE.
 7. A PERMANENT METAL DIE FOR CENTRIFUGAL CASTING, HAVING AN INSULATING REFRACTORY COATING FORMED ON THE INNER SURFACE THEROF BY APPLYING TO THE DIE WHEN HEATED TO A TEMPERATURE OF ABOUT 700* F. TO ABOUT 1000* F. AND WHILE BEING CENTRIFUGED AN AQUEOUS COMPOSITION OF POWDERED REFRACTORY MATERIAL, A CLAY-TYPED BINDER AND A WETTING AGENT. 